Structural prosthetic devices such as bone plates, joint replacement stems such as femoral and shoulder stem type prostheses, and intramedullary rods are used to transmit loads between bones or bone segments. Such prostheses are subjected to bending, axial compression, and shear loads. Such bending loads usually produce moments applied about axes perpendicular to a neutral axis representing the locus of points through the centroids of cross-sections of the prostheses taken normal to the neutral axis which is generally along the length of the prosthetic device. Torsion is often also applied about the neutral axis.
Since structural prosthetic devices are generally made of metal which is relatively rigid compared to bone, these devices frequently transfer load between bones or bony segments in a manner which greatly reduces loading on the bone in certain regions thereby inhibiting proper healing of the bone. It is a known property of bone that bone must be subjected to loading or disuse-atrophy will occur. Such protection of the bone, typically called stress shielding, frequently results in disuse-atrophy of certain regions of the bone providing an unhealthy situation for fixation of joint replacement prostheses of fracture fixation prostheses. This is particularly true where metallic fracture fixation prostheses must be removed, as is now recommended by some experts in the field, to avoid release of metal corrosion products into the body. Such removal will eliminate the structural contribution of the metallic fixation prostheses leaving only weakened bone to resist loading and such removal produce fracture of the bone unless healthy healing has occured. It is often, therefore, desirable to limit the stiffness of structural prostheses against certain loading modes so that at least after initial healing, preferably, as much of the load as possible is transmitted between bone or bony segments by the bone or bony segments themselves and that the prostheses simply act to align the segments and connect the prostheses to bone.
Further, where fixation plates are used to align fractured or resected bone segments the stiffness of the plate can inhibit bone growth between bone segments. Thus, bending flexibility is desired in order to increase the load transmitted between segments thereby stimulating bone growth.
Several investigators have been experimenting with plastic and composite plates in order to minimize the excessive stiffness of structural prostheses. While the development of such composite of plastic structural prostheses has much potential, the body is a hostile enviornment and plastics and composites frequently are severely affected by aging and exposure to the enviroment within the body. Further, there is limited knowledge about how plastics and composites capable of supporting the needed loads would behave in the body over long periods of time and what the effects of the corrosion products resulting from exposure of these materials to body fluids under the action of stresses would be. The bio-compatibility or the ability of the body to tolerate such plastics is not well understood. Conversely, metals (in particular cobalt chromium and titanium alloys) have been used in the body for several decades and they are found to be fatigue resistant, relatively corrosion resistant, and well tolerated by the body. Thus, it is advantageous to use these materials at least until such time as plastics and composites are proven to be effective in such use.
A principal advantage titanium and its alloys for use in femoral stems arises from the fact that these alloys are substantially less rigid than cobalt-chromium or other alloys used in this application. This increased flexibility results in lower stem loading, reducing stem stresses, and higher loading of the bone, reducing disuse-atrophy. Unfortunately, the notch sensitivity of surgical titanium alloy and the attendant drastic reduction in fatigue strength resulting from the use of a metallic porous surface on this material appears to limit the usefulness of porous coated titanium alloys. Thus, titanium alloys may not be well adapted for the exploitation of the substantial benefits available from the use of such coatings for both cement and biological fixation, but is a material of choice where such coatings are not used. On the other hand, cobalt chromium alloys do not appear to be notch sensitive, and seem well adapted for such exploitation.
The stiffness of structural prosthetic devices or fixation prostheses can, of course, be affected by the design of the prostheses itself. However, strength and geometric requirements (for example the need for a femoral stem to fit adequately into a femoral intramedullary canal) often limit the ability to provide as flexible a structural prosthesis as one would like. The use of more flexible metals such as titanium is also advantageous in this application since they can produce additional prosthesis flexibility. However, even with the use of titanium there are instances where a still more flexible metallic material would be preferred. Furthermore, many orthopaedic surgeons prefer to use cobalt chromium alloys since there is much more experience with the use of these alloys in the body than there is with titanium. Thus, there is a further need in the structural prosthetic device art for bio-compatible materials, in particular metals, of increased flexibility.
In some instances such as the use of bone plates for fracture fixation, increased bending and axial flexibility are desirable but increased torsional flexibility is undersirable since torsional stiffness helps resist movement between bone segments thereby stabilizing the fracture and thus promoting healing.
It has been discovered that single crystal materials developed for use a turbine blades in jet engines offer the potential for providing such increased flexibility in preferred directions while simultaneously providing increase stiffness in other preferred directons. The manufacture of parts using these alloys is described in a chapter entitled "The Development of Single Crystal Superalloy Turbine Blades" by M. Gell et al. in a book entitled "Superalloys 1980" which is a publication of the Proceedings of the Fourth International Symposium on Superalloys published in 1980; the article is found on pages 205-215 of the book which is published by The American Society for Metals. Crystals are anisotropic with respect to material properties. Stiffness and strength depend on direction. Cubic crystals, for example, have substantially lower stiffness in the &lt;001&gt; direction. Metals conventionally used for structural or fixation prostheses are polycrystalline in nature and generally have equiaxed grains. Such materials exhibit isotropic properties since the grain and crystal orientations are random and the stiffness is thus intermediate in value between the minimum and maximum crystal stiffness values.
Analogous technology to the single crystal materials is the development of directionally solidified materials or polycrystalline columnar grain structure. Such directionally solidified alloys are discussed in an article by Francis L. VerSnyder and N.E. Shank entitled "The development of Polymer Grain in Single Crystal High Temperature Metals Through Directional Solidification" appearing in Material Sicence and Engineering, Vol.6, No.4, 1970, pp. 213-247; such alloys are also discussed in U.S. Pat. No. 3,677,835 noted above.